Monitoring systems for measuring the respiration activity of a subject are known for various applications. For example, in clinical intensive care situations, such applications are used to monitor the respiration activity of a patient. Another application is a biofeedback system based on guided breathing exercises to assist a subject to fall asleep. Moreover, a further application for a method and a device for monitoring the respiration activity is the early detection of the drowsiness of a driver of a vehicle. In all these applications sensors are used to acquire a sensor signal representing the respiration activity of the subject of interest, and this signal is further processed and interpreted by the system.
A measuring principle to monitor the respiration effort is thorax inductive plethysmography, where a band is placed around the subject's thorax and monitors the change of the cross sectional area of the thorax due to breathing. Although such a so-called respiband is commonly used in medical applications, however, it is not acceptable in consumer product applications, like the above mentioned biofeedback systems or driver monitoring systems, since the attachment of the band is inconvenient, cumbersome and not acceptable for the common user. For this reason contactless methods are preferred. Sensor based monitoring systems exist that comprise an array of contactless sensors such as radar sensors based on the Doppler-Radar principle. Each of these sensors is able to detect a change of a distance during a time period of an object from the sensor. Due to their operational principle Doppler-Radar sensors can be used for detecting a breathing related thorax motion. They can readily be integrated into furniture parts or in car equipment such as a car seat or the steering wheel. For example, for the above mentioned application of early detection of a driver's drowsiness, an array of Doppler-Radar sensors can be integrated into the backrest of the drivers seat. By such an array a contactless reliable monitoring of the breathing activity is possible.
In the use of the well-known method of inductive plethysmography, as mentioned above, the signal processing can be performed on the basis of a vast experience, however, the method of contactless monitoring the respiration activity with the help of Doppler-Radar sensors is more difficult because of a lack of such experience. An important problem in this context is that the algorithms developed for respiband measurements cannot be readily transferred to Doppler-Radar sensor measurements and still have to be developed to provide processing results with the same reliability. Generally there is a desire to apply the experiences with well-known measurement techniques, like inductive or resistive measuring, to the field of Doppler-Radar measurements.